The present invention relates to an element which is to be mounted on the shaft of a non-positive displacement machine and which forms a gap for limited passage of a flowing medium, having a bush-like part which is to be mounted on the shaft of the non-positive displacement machine and whose outer periphery has a recess for a ring, the ring which is partially surrounded at its end faces by circularly annular walls of the recess during operation of the non-positive displacement machine    a) forming an axial gap in each case with the circularly annular walls of the recess and forming a radial gap with the bottom of the recess,    b) being held via frictional contact by a rigid wall surrounding the ring.
Somewhat similar elements are disclosed in international patent application no. WO 02/12728 (=DE 100 38 586) and published US patent application no. US 2002/125649 (=DE 100 62 204). In these documents, the element has served as a relief piston in an axial thrust balancing device or as a sealing bush in a shaft seal. A common feature of both applications was that a slotted resilient ring was used which was widened slightly during operation of the non-positive displacement machine by the prevailing pressure, pressed against the housing inner wall and fixed in such a way.
The present invention, however, is not aimed only at elements with slotted rings, but is intended also to be suitable for application in closed rings. Independently of their design, rings made from a soft, flowable material result in the problem that their edge zones on the one side are pressed gradually into the gap between the bush attached to the shaft and the surrounding housing hole by the pressure which acts on them. This results in increasing wear of the ring and the risk of the sealing action diminishing.